I had a flight school. The Insurance is just part of the rental rate. The trick is to figure your minimum rental hours per month (You are sure you will fly) and divide all your costs into that number of hours. I flew 100 hours a month with a 150 and enjoyed the experience.
The most important thing you can do is NOT get wrapped up in the BIG picture mentality.
You need to have someone that can teach ground school like the pied piper and get people fired up about flying. Your instructors must LIKE dealing with the public. If you run across a wannabe airline pilot, show them the door. The instructor will make or break you.
Maintaining the aircraft is very important in customer appeal and confidence. If it looks bad, it is bad. Containing your maintenance costs is a very tricky deal. It helps to be able to wrench and know just how much you can do for yourself. Hello to 100 hour inspections. Be associated with a shop that will give you a good deal on space and maintenance. They need aircraft owners and students grow up to be owners.
Be optimistic and infect those around you with the enjoyment of aviation. Never let a face come through your door that leaves a stranger. The hardest step for a student pilot is opening that door for the first time.
Fly and have fun.

I kinda had one for a while. Would recommend you stay away from 141, in my opinion, it's not worth the paperwork and hassle. The bigger the business got the more of a pain in the butt it was an the less money I made. I'd consider doing it again if it was kept to a small, one plane or two plane, operation....or if I could figure out how to make a dime. It's nice to have a mechanic that will trade flying for wrenching. I'd use free lance CFI's to keep your paperwork down.